During semiconductor wafer processing such as deposition, epitaxial growth, etching and the like, precise control over the concentration of each employed processing gas is required. Depending on the particular process, processing gas concentration can control deposited film thickness and doping level, epitaxial film stoichiometry, etch rate, etc. Additionally, precise processing gas control must be maintained over many processing runs to prevent wafer-to-wafer variations and to allow semiconductor device fabrication in mass.
Processing gas concentration normally is controlled via a flow regulator, such as a mass flow controller. The flow regulator controls processing gas concentration within a processing chamber by controlling the processing gas flow rate into the processing chamber. As used herein, a processing gas includes gasses, vaporized processing liquids and the like, and a processing gas flow regulator includes any flow regulator (e.g., flow meter, mass flow controller, etc.) that affects processing gas flow rate (e.g., a gas flow regulator, a processing liquid flow regulator, etc.).
To ensure accurate flow rates, and thus accurate processing gas concentration, typically each processing gas flow regulator is subjected to a one-time, labor intensive calibration procedure. For example, a conventional calibration procedure for a deposition process might include:
1. selecting a "best guess" flow regulator setting for a desired film thickness; PA1 2. depositing a film with the selected flow regulator setting; PA1 3. measuring the film thickness actually deposited; PA1 4. adjusting the flow regulator setting to compensate for any error between the actual film thickness and the desired film thickness; and PA1 5. repeating steps 1-4 until the actual film thickness is within an acceptable tolerance of the desired film thickness.
Once the proper flow regulator setting is determined, the flow regulator is deemed calibrated, and subsequent flow rates are selected based on the determined flow regulator setting.
Because the calibration procedure is a lengthy, iterative process involving substantial chamber downtime, flow regulators typically are calibrated in the manner described above only once. Thereafter, a less precise re-calibration process is employed periodically (e.g., daily).
One conventional method for re-calibrating a production chamber's flow regulator is to select a standard processing gas flow rate with the regulator and then to measure the rise in chamber pressure that results from the selected flow rate. This pressure rise is compared to an expected pressure rise measured on a laboratory chamber having a flow regulator of known accuracy. Any deviation between the production chamber's pressure rise and the laboratory chamber's pressure rise is attributed to an inaccuracy within the production chamber's flow regulator, and the calibration of the production chamber's flow regulator is adjusted until the production chamber's pressure rise matches the laboratory chamber's pressure rise.
A problem exists with such conventional re-calibration procedures because they are based on the assumption that, but for an inaccuracy in the production chamber's flow regulator, the pressure rise within the production chamber will equal the pressure rise within the laboratory chamber. However, this assumption is flawed because the assumption neglects other reasons for a difference in pressure rise between the production chamber and the laboratory chamber. Different chamber specifications between the production chamber and the laboratory chamber (i.e., "between-chamber" variations) such as different pump speeds, different throttle valve leakage rates and different foreline conductances can cause the pressure rise within the production chamber to differ from the pressure rise within the laboratory chamber.
Additionally, variations within the production chamber itself (i.e., "in-chamber" variations) such as within tolerance changes in leak rate, pump speed, throttle valve leakage, degassing/absorption properties, etc., can cause the production chamber's pressure rise to differ from the laboratory chamber's pressure rise. However, conventional re-calibration procedures treat any difference in pressure rise as a production chamber flow regulator inaccuracy, and thus often result in erroneous adjustments in flow rate. When the flow regulator is erroneously re-calibrated, processing gas concentration within the production chamber is altered and actual processing results deviate from desired processing results (e.g., different deposited film thickness or doping level, improper stoichiometry, over/under etching, etc.)
A need therefore exists for a procedure for ensuring flow regulator calibration independent of both between-chamber variations and in-chamber variations that affect pressure rise. Such a procedure should be economical and expedient, requiring minimal chamber downtime